The present invention relates to a system for controlling injection of fuel for an internal combustion engine with a direct fuel injection system where a combustion process is changed from a mode to another mode as well as the quantity of injected fuel, and more particularly to the system for smoothly controlling power characteristics when a combustion mode is changed.
In the direct fuel injection system for the engine, highly pressurized fuel is directly injected into a cylinder of the engine during a compression stroke and then mixture is ignited by a spark plug. The applicant of the present invention has proposed the fuel injection system where fuel injection is advanced so as to provide a homogeneous charge and retarded to provide a stratified charge. The system is so designed that the combustion process changes from a stratified charge combustion to a homogeneous charge combustion at a point where the engine torque obtained through the two combustion modes smoothly changes.
However, atmospheric conditions such as atmospheric pressure, temperature and humidity change, or elements of the fuel injection system deteriorate with time. As a result, the actual engine torque differs from the theoretically obtained engine torque, so that the engine torque changes stepwise at the combustion mode changing point, thereby impairing smooth driving.
Japanese Patent Application Laid-Open 60-36719 discloses a fuel injection system where density of intake air is detected. When the intake air density increases, the point where the stratified charge combustion is changed to the homogeneous charge combustion is shifted to a light engine load side. The point is shifted to a heavy engine load side in case the intake air density decreases. However, the system requires an intake air density sensor, and in addition, cannot cope with the problems caused by the deterioration with time. Although the combustion efficiency may be improved, smooth engine torque variation cannot be obtained because the combustion changing point of the combustion process is shifted by the control operation of the system. The combustion mode changing point is set at a point where an optimum combustion efficiency may be obtained. Therefore shifting the combustion mode changing point affects the combustion efficiency of the engine.